1. Field of the Invention
The present invention is generally related to curable silicone compositions. More particularly, the present invention is related to organosiloxane copolymer compositions which cure to an elastomer having superior optical resolution, excellent folding recovery and high refractive indices, making them particularly suitable for fabricating foldable intraocular lenses.
2. Description of Related Art
Silicones are used extensively for applications in which optical quality materials having resilient characteristics are preferred. A particularly suitable application for optically clear silicone elastomers is in the fabrication of lenses which are surgically implanted in the eye as a replacement for the natural lens. Known as intraocular lenses, these are most frequently implanted subsequent to cataract surgery which results in the extraction of the natural lens. Advantageously, silicone is a suitably inert material and is well tolerated in the eye. Moreover, when carefully formulated, silicone based compositions can be prepared which have suitable mechanical strengths and sufficiently high optical resolution for use as a human prosthetic lens.
Intraocular lens implant procedures involve preparing an incision in the eye, passing the lens into the eye, and then suitably anchoring the lens within the eye so that it does not subsequently become dislodged. Procedures in which non-resilient lenses are implanted, for example, polymethylmethacrylate intraocular lenses, require incisions which are at least as long as the lens diameter or 6-8 mm. During the past decade small incision surgery has become increasingly popular among ophthalmic surgeons. These techniques involve implanting foldable intraocular lenses through incisions as little as 2 mm. Because silicone elastomer intraocular lenses can be folded or rolled into configurations having a reduced cross-sectional diameter, these types of lenses have grown in popularity.
Foldable intraocular lenses are preferably as thin as possible to provide reduced folded cross-sectional areas. Thinner lenses are also much easier to fold or roll into a configuration which is easily inserted. A problem associated with silicone based intraocular lenses is the inherently low refractive .index of the most common and strongest polyorganosiloxane, polydimethylsiloxane (n.sub.D =1.40). As a result, lenses prepared from such materials having a low refractive index are thicker than lenses having the same degree of sight correction prepared from a higher refractive index material. Accordingly, optically effective polydimethylsiloxane based intraocular lenses are typically too thick for conveniently folding or rolling into shapes having minimum cross-sectional area.
In order to fabricate thinner foldable silicone intraocular lenses some lens manufacturers have provided silicone compositions with increased refractive indices. For example, a higher refractive index silicone has been prepared by replacing at least part of the dimethylsiloxane with the much weaker diphenylsiloxane which has a much higher refractive index. However, in order for the polyorganosiloxanes of dimethylsiloxane and diphenylsiloxane to provide optically clear cured elastomers, the crosslinking reagent and filler must match the refractive index of the polyorganosiloxane.
Typical organosiloxane crosslinking reagents, such as copolymers of methylhydrosiloxane and dimethylsiloxane, have low refractive indices. When these copolymers are utilized in the cure of diphenylsiloxane copolymers, the final cured elastomer is found to be hazy or even opaque. Similar problems are raised if the refractive index of any filler used is not matched to the refractive index of the polyorganosiloxane.
Furthermore, there are other major problems associated with intraocular lenses prepared from these elastomers, including poor optical resolution and the inability for these lenses to recover sufficient optical resolution once they are unfolded from a folded configuration. Optical clarity associated with a particular material does not necessarily guarantee good optical resolution and folding recovery.
Additionally, silicone based intraocular lenses are typically prepared with silicone based polyorganosiloxanes having a significant amount of low molecular weight material. The presence of the low molecular weight silicones can be responsible for significant lens shrinkage and weight loss.
Accordingly, it is an objective of the present invention to provide a curable optically clear silicone composition having a high refractive index.
It is also an objective of the present invention to provide a curable silicone composition useful for fabricating intraocular lenses having a low folded profile.
It is further an objective of the present invention to provide a curable silicone composition useful for fabricating intraocular lenses having superior optical resolution and folding recovery.